The invention is generally related to devices for sensing physiological parameters and, more particularly, to catheters having an ultrasonic device.
The heart beat in a healthy human is controlled by the sinoatrial node ("S-A node") located in the wall of the right atrium. The S-A n electrical signal potentials that are transmitted through pathways of conductive heart tissue in the atrium to the atrioventricular node ("A-V node") which in turn transmits the electrical signals throughout the ventricle by means of the His and Purkinje conductive tissues. Improper growth of or damage to the conductive tissue in the heart can interfere with the passage of regular electrical signals from the S-A and A-V nodes. Electrical signal irregularities resulting from such interference can disturb the normal rhythm of the heart and cause an abnormal rhythmic condition referred to as cardiac arrhythmia.
Electrophysiological ablation is a procedure often successful in terminating cardiac arrhythmia. This procedure involves applying sufficient energy to the interfering tissue to ablate that tissue thus removing the irregular signal pathway. However, before an ablation procedure can be carried out, the interfering tissue must first be located.
One location technique involves an electrophysiological mapping procedure whereby the electrical signals emanating from the conductive endocardial tissues are systematically monitored and a map is created of those signals. By analyzing that map, the interfering electrical pathway can be identified. A conventional method for mapping the electrical signals from conductive heart tissue is to percutaneously introduce an electrophysiology ("EP") catheter having mapping electrodes mounted on its distal extremity. The catheter is maneuvered to place those electrodes in contact with or in dose proximity to the endocardium of the patient's heart. By monitoring the electrical signals at the endocardium, aberrant conductive tissue sites responsible for the arrhythmia can be pinpointed.
Once the origination point for the arrhythmia is located in the tissue, the physician may use an ablution procedure to destroy the tissue causing the arrhythmia in an attempt to remove the electrical signal irregularities and restore normal heart beat or at least an improved heart beat. Successful ablation of the conductive tissue at the arrhythmia initiation site usually terminates the arrhythmia or at least moderates the heart rhythm to acceptable levels.
The distal end of an EP catheter may include mapping electrodes as well as an ablation device for performing the ablation procedure. One type of ablation device includes an ablation electrode that emits radio frequency ("RF") energy to heat the target tissue to a temperature high enough to cause ablution of that tissue. Other types of ablution devices may be used and in the following disclosure, an ultrasonic device is disclosed.
As the ablation procedure progresses, heat is generated and the surrounding blood is exposed to this heat. At approximately 100.degree. C., charring and boiling of the blood take place. Charring is particularly troublesome at the surface of the ablation device because emboli may form on the surface of the device to an extent that the catheter must be removed and cleaned before the procedure can continue. Furthermore, in RF ablution procedures, charring and boiling of the blood seriously modify the electrical conductivity of blood and tissue and cause an increase in the overall electrical impedance of the electrical heating circuit and a drop in the power delivery to the tissue. Too great a rise in impedance can result in sparking and thrombus formation within the heart, both of which are undesirable.
Although no significant amount of heat is generated in the ablation device itself, adjacent heated endocardial tissue will heat the ablation device via heat conduction through the tissue. Because part of the active transducer is in contact with the blood in the heart, blood boiling, emboli development, and clotting can result if the surface temperature of the transducer exceeds 90.degree.-100.degree. C. If this occurs, the ablution procedure must be stopped regardless of whether the entire ablution procedure has been completed. The catheter must then be removed from the patient, the attached necrotic tissue removed, and the catheter reinserted into the patient. Such cleaning processes require extra time and unduly prolong the ablation procedure. To avoid such undesirable circumstances, a temperature sensor may be incorporated at the distal end of the catheter to monitor and maintain a selected temperature during ablation. The ablation process can then be controlled so that the temperature is not allowed to increase above a predetermined level.
Temperature sensors have been incorporated in catheters for some time. In some cases, these prior temperature sensors were mounted at particular points inside the catheter body. While perhaps suitable for some applications, such a mounting scheme is less desirable for EP applications. The surrounding body of the catheter will insulate the temperature sensor from the tissue heat and will also impose a delay in heat transfer. More time to stabilize will be required thus slowing sensor response time. The tissue and blood temperatures may actually be greater than the internal sensor indicates.
Another approach involves mounting temperature sensors externally on the catheter body. Some of these temperature sensors comprise thermocouples having elongated leads for sending temperature signals to the proximal end of the catheter. However, when a thermocouple and its leads are mounted to the outer surface of the catheter body, the diametric profile of the catheter body is undesirably increased and in some cases, difficulty is encountered in securely mounting such a structure externally. In the case where the EP catheter utilizes an ultrasonic transducer for ablation, it is more difficult to mount a temperature sensor in the transducer itself. Ultrasonic transducers are typically composed of relatively brittle piezoelectric crystalline material that is somewhat fragile and fracturable. Attempting to mechanically drill through the crystalline structure can cause fractures in the structure that may leak body fluids into the catheter interior, or may eventually cause the crystal itself to fail. This characteristic has made it difficult to mount temperature sensors at the surface of the ultrasonic transducer and instead, such sensors have typically been mounted at a position proximal to the crystal. This of course delays the sensor's response to temperatures located at the distal tip of the catheter and for that reason, is undesirable.
When used in a percutaneous ablation procedure, it is sometimes difficult to closely control the orientation of the distal end of the catheter. If a "side-fire" type of ablation device is used, only a pan of the circumference of the device will be in contact with the tissue to be ablated. If the distal end should include only a single temperature sensor, that sensor may provide an accurate temperature indication if it is in contact with the tissue. However, if the distal end of the catheter is oriented such that the sensor is positioned in the cooler flowing blood, its temperature indication may not be as high as the temperatures of other pans of the ablation device. Blood boiling, emboli creation, and clotting may actually be occurring while going undetected.
A similar situation may occur in the case of an "end-fire" ablation device. If the temperature sensor is located at a position removed from the ablation device, it may indicate a temperature lower than the hotter device.
As used herein, a "side-fire" device is one that is mounted such that it conducts energy sideways in relation to the catheter shaft. An "end-fire" device is one that is mounted such that it conducts energy at the distal end of the catheter in relation to the catheter shaft.
A further problem encountered in working with ultrasonic transducers is their tendency to "pump" fluids through any openings associated with them. The physical movement of the transducer as it is performing its transducing function on the tip of a catheter for instance, can cause body fluids to enter the interior of the catheter through openings in the transducer unless careful sealing measures have been used to seal the transducer openings. The entrance of body fluids can cause a damping effect on the ultrasonic crystal causing it to become less effective in providing ablation energy.
Hence, those skilled in the an have recognized the need for a temperature sensing device or devices mounted at the distal end of an electrophysiology ablation catheter that is configured to provide a more rapid response time and accurate temperature indication. The ultrasonic transducer and catheter should provide a mounting configuration that provides for securely mounting the transducer to the distal end of the catheter tube without any undue ultrasonic damping. Such mounting configuration should provide for an effective seal so that body fluids do not enter the catheter and affect the operation of the transducer. Additionally, these temperature sensors should be relatively small so that the sensor does not interfere with the effective operation of the ablation device and response time is shortened. In addition, the temperature sensor should be relatively inexpensive to manufacture and reliable in use. The present invention fulfills these needs and others.